The present invention generally relates to chemical vapor deposition methods, and more particularly to a chemical vapor deposition method which is employed in a chemical vapor deposition apparatus of a type which makes a plasma self-cleaning using a gas which includes fluorine.
Recently, a high throughput is required of a chemical vapor deposition (CVD) apparatus. For this reason, a plasma self-cleaning is used within a chamber of the CVD apparatus so as to minimize the down time of the CVD apparatus.
In the conventional CVD apparatus, the plasma self-cleaning is made by use of a gas which includes fluorine. Thereafter, prior to forming a CVD layer on a wafer, the inside of the chamber is coated with a material identical to that of the CVD layer to a thickness of several microns so as to suppress the effects of the fluorine gas remaining within the chamber after the plasma self-cleaning.
However, in the case where the CVD layer is a SiO.sub.2 layer or a phospho-silicate glass (PSG) layer, it is impossible to eliminate or sufficiently suppress the gas which includes CF.sub.4, SF.sub.6, NF.sub.3 and the like and is used for the plasma self-cleaning. For this reason, fluorine is inevitably mixed within the SiO.sub.2 or PSG layer which is formed on the wafer.
A description will be given of an example of a case where a SiO.sub.2 layer is formed on a Si wafer by the CVD. First, a plasma self-cleaning is carried out using a NF.sub.3 gas. Thereafter, prior to forming the SiO.sub.2 layer on the Si wafer, a SiO.sub.2 layer is coated to a thickness of 3 microns inside the chamber of the CVD apparatus. Then, the SiO.sub.2 layer is formed on the Si wafer by the CVD.
FIG. 1 shows an analysis result of a secondary ion mass spectrometry (SIMS) which is made with respect to the SiO.sub.2 layer which is formed on the Si wafer by the CVD. As may be seen from FIG.1, fluorine of a high concentration is mixed into the SiO.sub.2 layer.
Therefore, according to the conventional CVD method, there are problems in that fluorine mixes into the SiO.sub.2 layer, PSG layer and the like which are formed on the wafer by the CVD and the quality of the layer deteriorates.